Why grebes have a ‘weed ceremony’

Great Crested Grebe - Podiceps cristatus. Picture: John Harding/BTO
Great Crested Grebe - Podiceps cristatus. Picture: John Harding/BTO
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A sure sign of the approaching spring is the sight of great-crested grebe pairs enacting their elaborate breeding displays.

These birds use shallow lakes for breeding, with some pairs even using those waterbodies located well within the county’s larger conurbations; ponds are avoided because great-rested grebes need a long take-off run in order to get airborne.

Suffolk was one of a handful of counties to retain its breeding grebes in the mid-1800s, when the fashion for ‘grebe furs’ reduced the UK population to as few as 32 pairs. Since then the population has recovered, benefiting from the cessation of persecution and the creation of new waterbodies, as old gravel pits have been flooded for amenity and wildlife benefit.

Grebe pairs form during mid-winter, although they have a long and not always successful ‘engagement’ period. Much of the courtship display is centred on strengthening the pair bond, and a number of distinct courtship behaviours are recognised. One of those most often seen is the head-shaking display, in which the head plumage is fanned into a ruff; the birds then face each other and shake their heads from side to side. The display is seen predominantly during the early stages of courtship and is usually performed when the birds are reunited after a period of separation, suggesting that it is used as a greeting and for reinforcement.

The most elaborate of the great-crested grebe’s displays is the ‘weed ceremony’ which, as its name suggests, takes place just before the pair begin to build their nest platform, which is made of waterweed. As part of this ceremony the two birds make a slow and deliberate dive to collect weed, before returning to the water’s surface and swimming towards each, their heads held low to the surface. As they meet, the birds rise from the horizontal to adopt a rigid vertical posture, which they hold by paddling their webbed feet rapidly, treading the water.

Although the courtship displays announce the presence of the breeding grebes, the birds become more retiring once the first egg has been laid. When incubating, the sitting bird (both members of the pair do their share) will slip off the nest if it spots a distant predator. If a bird has more time when leaving the nest then it will cover the eggs with waterweed, which is why the eggs quickly change colour from white to a muddy brown. This reduces the chances of the eggs being spotted by a predator. The resulting chicks would also be at risk were they to spend long in the nest, which is why they leave the nest with their parents within a few hours of hatching. At this young age the parents often carry the small chicks on their back, which can make them surprisingly difficult to spot.